On page 13 of the attached link, under the heading "Synchronous Operation", paragraph two is the sentence "Both grounds must be connected on the PCB at only one point, ideally close to the GND pin." What does this mean in terms of board layout? On page 17 is a picture of a recommended board layout which appears to show both GND and PGND connected to the same ground pad that the part's exposed thermal pad is soldered too. What am I supposed to be avoiding here in terms of PCB layout?

You are avoiding ground lift. This is when the voltage on the ground line is not zero but some higher value. This is caused by currents flowing through the wires causing small voltage drops. This is avoided if all the grounds are connected at a single point rather than being chained.

They are basically talking about a similar idea to a "star" grounding arrangement, where you want to avoid inductive ground loops which can cause noise, oscillations, and magnetic-field inductive transmission. Since the ckt is using an inductor as its major component, you don't want it inductively coupling willy-nilly [FYI, a rigorous engineering term] to any other part of the ckt, such as an inductive ground trace.

Strictly speaking, a ground shift problem involves the current through a trace or wire causing a voltage shift due to the resistance of said trace|wire, which appears on the groundof another ckt which is wired in a daisy-chain fashion off [ie, downstream] of the first ckt.Simple V = I * Rwire. Also, at high-frequencies, the inductance [impedance] of said trace|wirealso becomes a factor, ie V = I * Zwire. So, the star wiring arrangement is intended toeliminate such daisy-chain wiring.

Say you have an ADC chip that uses ground as a reference. You run the ground signal along a trace which joins a trace that has a high-current device on it. The combined trace then continues to the power supply ground.

Since the trace has resistance, when the high current device is operating it will cause a voltage drop along the combined trace which will lift the ground to the ADC by I*R, throwing off the ADC results.

The spec sheets for A/D converters usually talk about using separate groundingsystems [ie, ground planes] for digital and analog cktry, and also using a one-pointground connection between the 2 planes, and also never running digital traces intothe analog areas. Therefore, with good design, the ADCs will never see a ground shift associated with high-current devices.

I did that once, grounded a controller up stream of a 10A DC switch and everything else to the battery. Reset the controller...The Good News was that it took a 1" piece of wire.. The Bad News was that I had to do it. I moved a part a tenth and re-poured the ground plane to do it.I didn't think the change was serious enough to run the DRC again..

Bob

--> WA7EMS "The solution of every problem is another problem." -Johann Wolfgang von GoetheI do answer technical questions PM'd to me with whatever is in my clipboard

Some pcb layout programs do the pour automatically, after all the traces have beenlaid out, so you don't need to do it manually. Plus, the programs should be smart enough that they don't accidentally short everything out in the process.